Method for producing magnesium ferrosilicon



March 31, 1970 R A. HARD- ETAL 3,503,857

METHOD FOR PRODUCING MAGNESIUM FERROSILICON Filed April 24, 1967 INVENTORS ROBERT A. HARD BY D ALD J. HANSEN A ORNEY United States Patent US. Cl. 204-71 Claims ABSTRACT OF THE DISCLOSURE Method for preparing magnesium-containing ferrosilicon by electrolyzing magnesium oxide which is suspended in a fused electrolyte which floats on cathodically energized molten ferrosilicon. Magnesium liberated during electrolysis is attracted to the cathodic molten ferrosilicon and dissolved therein.

This invention relates to the making of magnesiumcontaining ferrosilicon alloys. More particularly, this invention relates to an electrolytic process for incorporating magnesium in ferrosilicon alloys.

Magnesium-ferrosilicon is a material widely used in industry for introducing magnesium into molten iron and usually contains from about 3 to magnesium, about 40 to 50% silicon with the balance mostly iron. Very often the magnesium content ranges between about 5 and 10%. In the past, magnesium-bearing ferrosilicon alloys have been manufactured by plunging ingots of magnesium metal into molten ferrosilicon (about 50% Si) until the desired magnesium content was obtained. On account of the low boiling point of magnesium (1107 C.) as compared to that of molten ferrosilicon, a substantial amount of magnesium was invariably lost by volatilization which resulted in a significant economic penalty in view of the relatively high cost of magnesium. Moreover, considerable care must be taken in order to achieve the desired magnesium level in view of the variable loss of magnesium through volatilization.

It is therefore an object of the present invention to provide a method of making magnesium-containing ferrosilicon alloys whereby loss of magnesium is essentially eliminated.

It is another object of the present invention to provide a method of making magnesium-containing ferrosilicon which avoids the necessity for using magnesium metal as a starting material.

Other objects will be apparent from the following description and claims taken in conjunction with the drawing which shows, somewhat schematically, an electrolytic cell suitable for the practice of the present invention.

A process in accordance with the present invention comprises providing a molten bath of fused salt which is suitable for use as an electrolyte in the electrolysis of magnesium oxide and providing molten ferrosilicon beneath the molten salt bath and in contact therewith. An electric potential is applied to the molten ferrosilicon whereby it is cathodically energized and an anodically energized electrode is placed in contact with the molten salt bath. Magnesium oxide, in finely divided form, is suspended in the molten salt bath and electrolyzed whereby magnesium metal is liberated, at the molten ferrosilicon cathode, and dissolved therein.

Through the use of the foregoing process, magnesium oxide, which is relatively inexpensive in both cost and handling, as compared to magnesium metal, is used as the starting material, and there is negligible loss of magnesium.

The process of the present invention can be more readily understood by reference to the drawing which shows at 1 an electrolytic cell unit having steel shell 3 mounted on supports 5 and surrounding refractory insulation 7.

The alumina brick structure 9, as shown, serves as an electrically nonconductive vessel for containing molten ferrosilicon shown at 11, upon which a fused salt bath 13 floats due to its lesser density. The molten ferrosilicon is introduced into the cell which contains molten electrolyte and these material are maintained in this condition by the heating effect of the electric current which passes between electrodes 15 and 17 via graphite cell base 18. Supplementary heat can also be employed through the use of auxiliary alternating current heating.

As indicated in the drawing, the molten ferrosilicon 1'1 is in contact with electrode 17 via base 18 and is thus cathodically energized. Consequently, when ,sufiicient electric potential is applied to the cell 1, magnesium oxide, introduced at 19, and suspended in the fused salt bath 13, is electrolyzed. That is to say, magnesium metal is liberated at the cathodic molten ferrosilicon and dissolves therein thus producing a magnesium-ferrosilicon product. Oxygen is liberated during the electrolysis 'of magnesium oxide, at the anodes 15, which are made of carbon or graphite, and the oxygen combines therewith to form carbon monoxide which exists above the bath. Thus, the liberated oxygen is essentially prevented from reacting with and reoxidizing the liberated magnesium metal.

As electrolysis continues and magnesium is dissolved in the molten ferrosilicon, additional finely divided magnesium oxide is added to the fused salt bath to replenish that which was electrolytically decomposed. For a given cell of known current efiiciency, the rate of magnesium production, and hence the magnesium content of the cathodic ferrosilicon product can be readily calculated. Alternately, the magnesium content can be determined by periodic sampling and when the desired magnesium content is achieved in the molten ferrosilicon, the product can be withdrawn by reduced pressure siphoning, or by means of an exit port (not shown).

In the practice of the present invention, the composition of the starting molten ferrosilicon material should be at least about 30% Si and can be up to Si. At silicon contents of less than about 30% the amount of magnesium that can be maintained in -the molten alloy is very low and insignificant as a practical matter. The upper limit for the silicon content is determined by the relative density of the molten metal as compared to the fused salt bath electrolyte since it is essential that the molten metal be more dense so that the electrolyte floats above it.

Also, in the present invention, the cell operating temperature, i.e., the temperature of the molten cell contents, should be in the temperature range of about 1250 C. to about 1350 C. since at temperatures below about 1250 C. the magnesium-ferrosilicon product obtained begins to solidify whereas at temperatures above about 1350 C., the vapor pressure of magnesium tends to cause the formation of an unstable alloy product.

Control of the current density is also important in the present invention and the anode current density is preferably less than about 10 amperes per square inch in order to avoid polarization. While higher current densities can be used, cell efliciency decreases considerably.

Electrolytes, i.e., the fused salt baths, used in the present invention should be (a) molten in the temperature range of about 1250 C. to 1350 C. for the reasons previously noted, (b) have a density less than that of the molten ferrosilicon employed and (c) the cation constituents must be less noble than magnesium to avoid their reduction and deposition on the molten ferrosilicon in preference to magnesium.

The preferred fused salt bath, i.e., electrolyte, is one containing about 70% MgF and about 30% BaF Salts (III) Results:

Elapsed time from Percent mg. in start 01 cell operation I Tap No siphoned product (hr.) WhlCh are suitable, singly or in mixture, provided the fore- 7 8 13 going criteria are met, are shown in Table I. 2 11115 19.6 27 17.9 29.25 16.2 35 15.3 36 TABLE I 14.6 37. 5

Material Melting point, O. Specific gravity 1 3 6 EXAMPLE 3 1 38 2:; Using a cell unit generally similar to that shown in the 1,360 drawing having inner cross-section dimensions of about In the practice of the present invention, it has been found that the magnesium oxide starting material should be of a size that it is essentially suspended in the fused salt bath to avoid significant settling. A sizing of 200 mesh and finer (Tyler series) has been found to be suitable.

The following examples will further illustrate the present invention.

EXAMPLE 1 Using equipment generally similar to that shown in the drawing molten ferrosilicon containing about 50% Si is introduced into the cell which contains a molten mixture of 70% MgF and 30% BaF The ferrosilicon being more dense than the fused salt mixture settles to the bottom of the cell where it contacts the negatively energized graphite bottom element. The molten fused salt floats on top of and covers the molten ferrosilicon and is contacted by vertically positioned graphite electrodes. The electric potential across the cell is adjusted to provide a cathode current density of 4 to 5 amps/in. which maintains the molten state cell of the constituents. For a cathode area of 1 square foot the total cell current is on the order of 575 to 720 amperes and to maintain relatively constant operation, magnesium oxide (200 mesh and finer) is added to the fused salt bath at a rate of about 0.58 to 0.72 pounds per hour. Under the foregoing conditions magnesium metal is liberated and dissolves in the molten ferrosilicon at a rate of about 0.35 to 0.43 pound per hour. The oxygen liberated during the electrolysis at the graphite anodes reacts to form carbon monoxide and exitsthe cell. Molten product, i.e., magnesium ferrosilicon, is removed from the cell, for example, by siphoning, when the desired magnesium level is reached.

EXAMPLE 2 Using a cell unit generally similar to that shown in the drawing having inner cross-section dimensions of about 18" x 27 and using two anodes magnesium ferrosilicon was prepared under the following conditions and with the noted results:

(I) Materials:

Anode5% inch diameter graphite Cathode-50%ferrosilicon (100 pounds) Electrolyte MgF (245 pounds)|BaF (105 pounds) (II) Operating conditions:

Molten material temperature1300 C.

Cell currentl500 amperes Cell ampere hours5 3,400

Cathode current density2.6 amps/in.

MgO addition (200 mesh and finer)1.2 pounds per hour (50% C.E.)

18" x 27 and using two anodes magnesium ferrosilicon was prepared under the following conditions and with the noted results:

(I) Materials:

Anode5% inch diameter graphite Cathode50'% ferrosilicon pounds) ElectrolyteMgF pounds)+BaF (60 pounds) (II) Operating conditions:

Molten material temperaturel300 C. Cell current-2000 Cell ampere hours-42,000 Cathode current density3.4 amps/in. Cathode current eificiency60% MgO addition (200' mesh and finer)2 pounds per hour (60% CE.) (III) Results:

Product-magnesium ferrosilicon containing 9.9%

For all examples, there was no noticeable loss of magnesium metal and the magnesium ferrosilicon product obtained was highly suitable for use in the treatment of iron.

What is claimed is:

1. A method for producing magnesium-containing ferrosilicon alloys which comprises:

(1) providing a molten bath of fused salt suitable for use as an electrolyte in the electrolysis of magnesium oxide,

(2) providing molten ferrosilicon beneath the molten salt bath and in contact therewith,.the density of the molten salt bath being less than that of the molten ferrosilicon,

(3) applying an electric potential to the molten ferrosilicon whereby it is cathodically energized,

(4) contacting the molten salt bath with an anodically energized electrode,

(5) suspending finely divided magnesium oxide in the molten salt bath, and

(6) causing an electric current to flow between the anodically energized electrode and the cathodically energized molten ferrosilicon :sufficient to reduce said magnesium oxide whereby magnesium metal is liberated and deposited on the molten ferrosilicon and dissolved therein.

2. A method in accordance with claim 1 wherein the molten fused salt is a mixture of two or more of the following:

3. A method in accordance with claim 1 wherein the molten fused salt bath is a mixture containing about 70% MgF and about 30% BaF 4. A method in accordance with claim 1 wherein the 1,564,139 112/ 1925 Saklatwalla 20471 temperature of the molten fused salt and molten ferro- 3,022,233 2/1962 Olstowski 204-71 XR silicon is in the range of about 1250 C. to about1350 C. 3,024,177 3/ 1962 Cook 2047-1 XR 5. A method in accordance with claim '1 wherein the 3,093,558 6/1963 Labounsky 204--70 molten ferrosilicon contains between about 30% and 90% i 5 JOHN H. MACK, Primary Examiner References Cited D. R. VALENTINE, Assistant Examiner UNITED STATES PATENTS Us cl XR 880,489 2/1908 Kugelgen et a1. 204 70 204 64 1,310,449 7/1919 Seward 204-71XR 10 

